This invention relates to techniques and apparatus for imaging sections of a body by transmitting ultrasonic energy into the body and determining the characteristics of the ultrasonic energy reflected therefrom and, more particularly, to an improved ultrasonic scanning technique and system for such apparatus.
Ultrasonic imaging techniques have become common in clinical diagnosis. Ultrasonic imaging is recognized as having a number of attributes. Ultrasound differs from other forms of radiation in its interaction with living systems in that it has the nature of a mechanical wave. Accordingly, information is available from its use which is of a different nature than that obtained by other methods and it is found to be complementary to other diagnostic methods, such as those employing X-rays. Also, the risk of tissue damage using ultrasound appears to be much less than the apparent risk associated with ionizing radiations such as X-rays.
Many diagnostic techniques using ultrasound are based on the pulse-echo method wherein pulses of ultrasonic energy are periodically generated by a suitable piezoelectric transducer. Each short pulse of ultrasonic energy is focused to a narrow beam which is transmitted into the patient""s body wherein it eventually encounters interfaces between various different structures of the body. When there is a characteristic impedance mismatch at an interface, a portion of the ultrasonic energy is reflected at the boundary back toward the transducer. After generation of the pulse, the transducer operates in a xe2x80x9clisteningxe2x80x9d mode wherein it converts received reflected energy or xe2x80x9cechoesxe2x80x9d from the body back into electrical signals. The time of arrival of these echoes depends on the ranges of the interfaces encountered and the propagation velocity of the ultrasound. Also, the amplitude of the echo is indicative of the reflection properties of the interface and, accordingly, of the nature of the characteristic structures forming the interface.
There are various ways in which the information in the received echoes can be usefully presented. One common form of display is the so-called xe2x80x9cB-scanxe2x80x9d wherein the echo information is of a form similar to conventional television display; i.e., the received echo signals are utilized to modulate the brightness of the display at each point scanned. This type of display is found especially useful when the ultrasonic energy is scanned transverse the body so that individual xe2x80x9crangingxe2x80x9d information yields individual scan lines on the display, and successive transverse positions are utilized to obtain successive scan lines on the display. The two-dimensional B-scan technique yields a cross-sectional picture in the plane of the scan, and the resultant display can be viewed directly and/or recorded. It is known that ultrasound is almost totally reflected at interfaces with gas. This has led to the use of coupling through a fluid such as water or the use of a direct-contact type of transducer. In my U.S. Pat. No. 4,084,582, there is disclosed a type of apparatus having a console which typically includes a timing signal generator, energizing and receiving circuitry, and a display/recorder for displaying and/or recording image-representative electronic signals. A portable scanning head or module, suitable for being hand held, has a fluid-tight enclosure having a scanning window formed of a flexible material. A transducer in the portable scanning module converts energy from the energizing circuitry to ultrasonic energy and also converts received ultrasound echoes back into electrical signals which are coupled to the receiver circuitry. A focusing lens is coupled to the transducer, and a fluid, such as water, fills the portable scanning module in the region between the focusing lens and the scanning window. A reflective scanning mirror is disposed in the fluid, and a driving motor, energized in synchronism with the timing signals, drives the scanning mirror in periodic fashion. The ultrasound beam is reflected off the scanning mirror and into the body being examined via a scanning window formed of a rigid material.
For a two dimensional B-scan taken with the described type of scanning head, the dimensions scanned are: (1) depth into the body, which varies during each display scanline by virtue of the ultrasound beam travelling deeper into the body as time passes; and (2) a slower transverse scan caused by the scanning mirror. The display is typically in a rectangular format, e.g. the familiar television type of display with linear sweeps in both directions. However, the transverse scan of the ultrasound beam itself, as implemented by the scanning mirror, results in a sector scan. For distances deeper in the body, the fanning out of the sectors results in geometrical distortion when displayed on a linear rectangular display. For example, the azimuth dimension in the extreme far field may be, say 2xc2xd times larger than the azimuth dimension in the extreme near field. Thus, the density of information on the left-hand side of a conventional left-to-right display is much higher than the density of information on the right-hand side of the display. In other words, what appear to be equal distances in the body on the left and right hand sides of the display are actually substantially different distances. A solution to this problem was offered in my U.S. Pat. No. 4,325,381, wherein the scanning window was in the form of an acoustic lens for converging the scan of the ultrasound beam incident thereon from within the enclosure. The acoustic lens thereby serves to reduce geometric distortion of the scan of the ultrasound beam. In an embodiment illustrated in the referenced ""381 Patent, the window/lens is formed of a rigid plastic material in a substantially plano-concave shape, with the concave surface facing the outside of the enclosure. In this embodiment, the window/lens is provided with a focal length of between one and two times the distance between the reflective scanning means and the window/lens. A focal length of about one and-a-half times the distance between the reflective scanning means and the window/lens is indicated as being particularly suitable for a functioning embodiment.
It is among of the present invention to provide improvement over prior art approaches.
It is also among the objects of the present invention to provide a technique and apparatus that will facilitate the availability of high-quality inexpensive and portable ultrasonic imaging equipment for medical applications.
By taking advantage of the computing power of a personal computer, it should be possible to make very inexpensive ultrasound scanners for medical applications. By doing this, ultrasound scanners can be used in the physician""s office as well as by medical technicians operating telemedicine facilities for remote diagnosis. In order to keep the cost low, the resolution high, and the size and complexity low, it would be desirable to use a system employing mechanically scanned ultrasound because these can be made with relatively large apertures. In the previously referenced ""381 Patent there is described a mechanical scanner that had a window lens or xe2x80x9clens membranexe2x80x9d at the skin surface. In the geometry described in that Patent, it was not possible to have the center of the scan to be at the focal point of the lens membrane. A form of the present invention overcomes this deficiency.
It has been found that it is generally preferable to vary the focus of the scanner rather than use dynamic focus. Variable focus maximizes the intensity and resolution at all ranges. The focus cannot be varied dynamically in transmit. In my U.S. Pat. No. 4,257,271, selectable focus was achieved using a tapped delay line and a transducer with multiple electrodes. In accordance with a further form of the invention, a variable focus technique is implemented mechanically rather than electronically. This provides a continuous variation in focus without requiring multiple electrodes on the transducer.
There are certain requirements for a scanner that has a Cartesian scan and that keeps the aperture (and consequently, the azimuth resolution) constant at all ranges. The path length from the transducer to the skin should be an integral multiple of the round trip distance the sound travels between transmit pulses. There are multiple reflections from the membrane and transducer since they are parallel to each other. This distance requirement places these multiple reflections at the membrane surfaces in the image where it cannot obscure useful information at greater depths. In previous designs, this distance was chosen to be equal to the depth of the longest range displayed by the system. Because of this, the scanning mirror had to be at a much shorter distance than this from the membrane. The focal length of the lens membrane cannot be made short enough at a reasonable scanning width to have its focal point at the center of deflection of the beam.
In a form of the present invention, a distance of twice the depth of penetration is chosen between the transducer and the membrane. The scanning mirror is placed at the depth of penetration before the membrane. The focal length of the lens membrane can now be made equal to the depth of penetration, which is also equal to the distance from the mirror to the membrane.
Embodiments of the invention are applicable to an apparatus for ultrasonically imaging sections of a body by transmitting ultrasonic energy into the body and determining the characteristics of the ultrasonic energy reflected therefrom, the apparatus including timing means for generating timing signals; energizing/receiving means operative in response to timing signals; and display/record means synchronized with the timing signals for displaying and/or recording image-representative signals from the energizing/receiving means; the apparatus having a scanning module that includes: a fluid-containing enclosure having an ultrasonically-transmissive window which can be placed in contact with the body; transducer means in the enclosure, coupled to the energizing/receiving means, for converting electrical energy to a beam of ultrasonic energy and for converting reflected ultrasonic energy to electrical signals; means for focusing the ultrasound beam emanating from the transducer means; reflective scanning means rotatably mounted in the enclosure in the path of the ultrasound beam for effecting scanning of the beam across the body via the window; the window comprising an acoustic lens for converging the scan of the ultrasound beam incident thereon from within the enclosure.
One form of the method of the invention includes the following steps: selecting a desired maximum depth of penetration of the ultrasound beam into the body; and providing distances between the transducer means and the reflective scanning means and between the reflective scanning means and the acoustic lens window that are each substantially equal to the selected maximum depth of penetration.
A further form of the method of the invention includes the following steps: providing the reflective scanning means with a controllable variable focus; and controlling the reflective scanning means to select the focusing of said reflective scanning means. In an embodiment of this form of the invention, the step of providing a reflective scanning means with a controllable variable focus comprises varying the shape of the reflective scanning means.
Another form of the method of the invention includes the following steps: providing a controllable variable focusing means in the enclosure in the path of said ultrasound beam; and controlling the variable focusing means to select the focusing thereof.
A still further form of the invention includes the following steps: providing, on the rotatable reflective scanning means, a plurality of ultrasound reflecting facets having different focusing characteristics; and storing the electrical signals obtained during the scan by each of the facets. In an embodiment of this form of the invention, the facets implement focusing at different depth ranges in the body, and the stored signals are range-gated out of storage for B-scan display of echo amplitude as a function of range.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.